Simulated and experimental analysis of PVDF membrane hydrophone low-frequency response for accurate measurements of lithotripsy shockwaves

Accurate measurement of pressure waveforms is important for characterization of medical ultrasound equipment. Calibration and measurements can be difficult for high amplitude fields such as those produced by lithotripters. Additionally, it was found previously that the response of membrane hydrophones is sensitive to water conductivity, particularly at low frequencies. The purpose of this study was to develop an electrical model of the PVDF hydrophone to explain this effect and test methods to make membrane hydrophones insensitive to conductivity. The model was compared with measurements of lithotripsy shockwaves by membrane and fiber optic (FOPH) hydrophones at different water conductivities over a range of 6 - 1300 muS/cm. The model simulated the waveform distortion caused by conductivity. It was predicted that water conductivity must be lowered to unreasonable levels to measure shockwaves accurately. We modified the membrane hydrophone to eliminate this dependency by changing the liquid surrounding the membrane and by adding capacitance in parallel with the membrane. After modification, the waveforms measured by FOPH and membrane hydrophones were identical. The results suggest that membrane hydrophones are capable of accurately measuring lithotripsy shockwaves.